media: ddbridge: check pointers before dereferencing

Fixes two warnings reported by smatch:

  drivers/media/pci/ddbridge/ddbridge-core.c:240 ddb_redirect() warn: variable dereferenced before check 'idev' (see line 238)
  drivers/media/pci/ddbridge/ddbridge-core.c:240 ddb_redirect() warn: variable dereferenced before check 'pdev' (see line 238)

Fixed by moving the existing checks up before accessing members.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Tested-by: Jasmin Jessich <jasmin@anw.at>
Tested-by: Dietmar Spingler <d_spingler@freenet.de>
Tested-by: Manfred Knick <Manfred.Knick@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>